As practiced commercially, fused silica optical members such as lenses, prisms, filters, photomasks, reflectors, etalon plates and windows, have been manufactured from bulk pieces of fused silica made in large production furnaces. Bulk pieces of fused silica manufactured in large production furnaces are known in the art as preforms, boules or ingots. Blanks are cut from boules or ingots, and finished optical members are manufactured from glass blanks, utilizing manufacturing steps that may include, but are not limited to, cutting, polishing, and/or coating pieces of glass from a blank. Many of these optical members are used in various apparatus employed in environments where they are exposed to ultraviolet light having a wavelength of about 360 nm or less, for example, an excimer laser beam or some other ultraviolet laser beam. The optical members are incorporated into a variety of instruments, including lithographic laser exposure equipment for producing highly integrated circuits, laser generation equipment, medical equipment, nuclear fusion equipment, or some other apparatus which uses a high-power ultraviolet laser beam.
As the photon energy, pulse energy and pulse rate of lasers increase, the optical members which are used in conjunction with such lasers are exposed to increased levels of energy. Fused silica has become widely used as the material of choice for optical members in such laser-based optical systems due to their excellent optical properties and resistance to light-induced damage.
Laser technology has advanced into the short wavelength, high energy ultraviolet spectral region, the effect of which is an increase in the frequency (decrease in wavelength) of light produced by lasers. Of particular interest are short wavelength lasers operating in the UV and deep LTV (DUV) and vacuum UV wavelength ranges, which include, but are not limited to, lasers operating at about 248 nm, 193 nm, 157 nm and even shorter wavelengths. Excimer laser systems are popular in microlithography applications, and the shortened wavelengths allow for increased feature resolution and thus line densities in the manufacturing of integrated circuits and microchips, which enables the manufacture of circuits having decreased feature sizes. A direct physical consequence of shorter wavelengths (higher frequencies) is higher photon energies. In such optical systems, fused silica optics are exposed to high irradiation levels for prolonged periods of time, and this may result in the degradation of the optical properties of the optical members.
It is known that such light-induced degradation adversely affects the optical properties and performance of the fused silica optics by decreasing light transmission levels, discoloring the glass, altering the index of refraction, altering the density, and increasing absorption levels of the glass. Over the years, many methods have been suggested for improving the optical damage resistance of fused silica glass. It has been generally known that high purity fused silica prepared by such methods as flame hydrolysis, CVD-soot remelting process, plasma CVD process, electrical fusing of quartz crystal powder, and other methods, is susceptible to laser damage to various degrees.
It has been reported that when silica glass is exposed to non-polarized or circularly polarized UV laser beam, usually in the peripheral area of the exposure light beam, additional birefringence (induced edge birefringence) is generated due to strain caused by laser damage, but in the center area of the light beam, there is usually negligible induced birefringence. Recently, a new phenomenon of laser damage to silica material has been observed: when the silica glass is exposed to linearly polarized deep UV laser beam, in addition to the induced edge birefringence, additional birefringence is induced in the center of the exposed area of the glass (“polarization-induced birefringence” or “PIB”). The induced birefringence, especially polarization-induced birefringence, is of particular concern to immersion lithography systems where a liquid fills the gap between the last lens element and the wafer in order to enlarge the numerical aperture of the lens system. In such immersion lithography systems, the polarization state of the UV radiation needs to be controlled, desirably linearly polarized. The induced birefringence in the glass alters the polarization state of the UV radiation, causing reduction of phase contrast and system resolution. Therefore, for deep UV and vacuum UV immersion lithographic systems, it is highly desirable that the glass material used in making the lens elements has low induced birefringence damage, especially a low polarization-induced birefringence, when exposed to linearly or elliptically polarized UV radiation, in addition to low light-induced wave-front distortion (“LIWFD”) and high transmission.
Therefore, there exists a need for a synthetic silica material having, inter alia, a low level of polarization-induced birefringence, a low level of light-induced wavefront distortion, a high level of initial internal transmission, and method of making the same.
The present invention satisfies the above described needs for synthetic silica glass for use in lithographic applications.